Heat-Resistant Alloy Selection - The Importance of Microstructure Under Cycling Conditions
In the search for the lowest cost, some important details may be overlooked. Although the chemistries may be the same, alloys from different sources may perform much differently in service.
In practice, quenching fixtures are not subject to dead loads but rather to a combination of mechanical loads and cyclic thermal strains. Thermal-fatigue damage in a coarse-grained bar can lead to more significant distortion and damage in service than the use of a finer-grained alloy with lesser creep-rupture strength.
Cast and Wrought StructuresCase in point is that cast alloys, which typically have a very coarse grain structure, offer superior structural strength at red heat as compared to a finer-grained wrought product. They do, however, commonly suffer from cracking when subjected to cyclic duty due to their coarse grain structure.
Figures 1-4 compare cast and wrought grids used at a wire processor that provides spheroidized, normalized and annealing services. Traditional cast HT alloy grids were used in their furnaces. Their process involves the heating of steel-rod coils in a nitrogen atmosphere to approximately 1500°F followed by a controlled cooling.
In contrast to cast HT, this wrought alloy has a carbon content of 0.05% and is also manufactured to control its grain size, which greatly enhances resistance to cracking from thermal cycling. RA330 is also immune to sigma-phase formation. As a result, even after long-term exposure at 1500°F, RA330 alloy retains its ductility. This allows for life extension through restraightening and/or weld repair. After six years in service, the fabricated grids are still performing well. Good ductility has allowed for occasional restraightening as required.
Grain Size Variation in Wrought AlloysOver the years, alloy suppliers have looked for ways to reduce the production costs of these heat-resistant alloys by streamlining the production process. While the chemistry remains consistent, altering the mill production process can yield a rod coil product with a wide variety of microstructures. Some of these are suitable for high-temperature quenching, and others are not. Figures 5 and 6 show acceptable and unacceptable microstructures seen throughout the years. Experience shows that both the chemistry and the microstructure are critical to the performance of a wrought fabricated tray as well.
A study done on alloy-600 bar baskets fabricated using a combination of coarse-grained (ASTM 4) and fine-grained (ASTM 8-9) rods also showed the benefit of fine grains in quenching service. In this study, the baskets were evaluated after one year of service in a combination of 50% straight hardening at 1650°F, 30% carburizing at 1650°F and carbonitriding at 1500-1600°F. All cycles included an oil quench. It was found that carburization of the bar baskets was 0.022-0.043 inches deep in the fine-grained bars versus 0.050-0.060 inches in coarse-grained samples. Thermal-fatigue cracks were twice as deep in the coarse bars as the fine-grained bars on average.
J.C. Kelly likewise indicates in his experience that grain sizes finer than ASTM 4 are desirable for thermal-fatigue resistance in quenching fixtures. A test was conducted with corrugated boxes where half the box was constructed of material with a grain size of ASTM 4 and the other half was ASTM 00. After 18 months of service in a carbonitriding operation at 1650°F followed by an oil quench, the corners of the box cracked in the coarse-grained product while no cracking was evident in the finer-grained product.
Similar support for the benefit of fine-grained material in quenching service can be derived by considering the markets for two nickel alloys. RA330 at 35% nickel and 19% chromium is fairly similar in chemistry to alloy 800H/AT (32% Ni, 20% Cr). RA330 is common to the heat-treating industry, whereas 800H/AT is commonly used in the petrochemical industry.
The difference between the two involves their processing at the mill level. RA330 is mill annealed at a more moderate temperature – typically around 1900°F – whereas 800H/AT is required to be solution annealed at 2100°F minimum. Higher annealing temperatures yield a coarse grain structure (commonly ASTM 4 or coarser) and maximize creep-rupture strength in the 800H/AT product. RA330 has a typical grain size finer than ASTM 4. As a result, 800H/AT is most suited to structural components of petrochemical furnaces where operation is continuous and temperature cycling is minimal. RA330, with its finer grain structure, is more suitable for the rigors of thermal shock encountered by fixtures in the heat-treat industry.
ConclusionsWhile chemistry plays an integral part in the performance of a heat-resistant alloy, the microstructure of a material, as shown here, impacts resistance to thermal fatigue. Understanding both will ensure the proper material and method of manufacture are selected for your application. Before substituting alloys of similar chemistry, it is suggested that you check with your alloy supplier to ensure that the products are designed for similar operating conditions. IH
The author wishes to thank Hyper Alloys for their assistance in compiling some of the photos and information used in this article.
For more information: Contact Jason Wilson, technical marketing manager, Rolled Alloys, 125 West Sterns Rd., Temperance, MI; tel: 800-928-9482; fax: 734-847-3915; e-mail: firstname.lastname@example.org; web: www.rolledalloys.com
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: thermal fatigue, creep-rupture strength, sigma phase, coarse grain, thermal shock